The present invention relates to the technical field of nitration, in particular the preparation of nitrated aromatic organic compounds (hereinafter referred to synonymously as “nitroaromatics”, “nitration products” or the like) and their purification after production.
The present invention relates in particular to a process for removing impurities (in particular unreacted starting materials, reaction by-products, nitrating acid and reaction products thereof, e.g. nitrogen oxides or nitrous acid, etc.) from crude nitrated products obtained in the nitration of nitratable aromatic compounds by treating the crude products, after removal of the residual nitrating acid, with a washing medium. In other words, the present invention relates to a process for purifying crude nitrated products obtained in the nitration of nitratable aromatic compounds after removal of the residual nitrating acid.
Furthermore, the present invention relates to an apparatus or plant for removing impurities from crude nitrated products obtained in the nitration of nitratable aromatic compounds after removal of the residual nitrating acid. The apparatus or plant of the invention is, in particular, suitable for carrying out the process of the invention.
Finally, the present invention relates to a production plant for nitrating nitratable aromatic compounds with subsequent purification of the nitrated products.
Aromatic nitro compounds such as nitrobenzene (MNB), mononitrotoluene (MNT), dinitrotoluene (DNT), trinitrotoluene (TNT), nitrochlorobenzene (MNCB), etc., which are prepared by reaction of a corresponding aromatic such as benzene, toluene, xylene, chloro-benzene, dichlorobenzenes, etc., with nitric acid, either directly or in the presence of sulfuric acid catalyst and water-binding agent, have to be subjected before further processing to multistage washing and additional purification steps in order to remove the impurities which are still dissolved or suspended in the crude nitroaromatics, e.g. sulfuric acid, nitric acid, nitrogen dioxide, nitrophenols, nitrocresols, etc., which can be present, for example, as mononitro, dinitro and trinitro compounds, and other oxidation products such as nitrobenzoic acids and degradation products from the decomposition of nitrophenols, or the unreacted aromatics or undesirable isomers, e.g. in the production of TNT, from the crude mixture of nitroaromatics.
The washing of the crude nitroaromatics in order to remove the dissolved and suspended acids of the nitration mixture, the nitrophenols and other acidic impurities which can still be extracted by the washing medium usually consists of three steps (see, for example, F. Meissner et al., Industrial and Engineering Chemistry, Vol. 46, pages 718 to 724 (1954); Ullmanns Enzyklopädie der Technischen Chemie, 4th edition, Vol. 17, pages 384 to 386; H. Hermann et al., “Industrial Nitration of Toluene to Dinitrotoluene”, ACS Symposium Series 623 (1996), pages 234 to 249, editors: L. F. Albright, R. V. C. Carr, R. J. Schmitt; U.S. Pat. No. 6,288,289 B1; EP 1 816 117 B1). Water is usually used as washing medium with washing usually being carried out as a liquid/liquid wash (i.e. at temperatures at which the nitroaromatic to be washed is present as liquid).
This three-stage wash usually comprises the following steps:    1. An acid wash with water to remove the dissolved and suspended mineral acids, e.g. sulfuric acid, nitric acid and nitrogen dioxide (“acid wash”).    2. A basic or alkaline wash in the presence of a base (“alkali wash”), e.g. sodium carbonate (soda), sodium bicarbonate, sodium sulfite, sodium hydrogensulfite, ammonia, sodium hydroxide, potassium hydroxide, etc. (see, for example, U.S. Pat. No. 4,482,769 A, U.S. Pat. No. 4,597,875 A or U.S. Pat. No. 6,288,289 B1), to remove the weakly acidic impurities dissolved in the crude nitroaromatic, e.g. the nitrophenols, nitrocresols, nitrobenzoic acids, degradation products from the oxidative decomposition of the phenols or of aliphatic or cyclic hydrocarbons, etc., e.g. oxalic acid, etc., or the unsymmetrical isomers in the case of TNT (“basic wash”).    3. A neutral wash to remove the residual traces of alkali and to further reduce the amount of impurities still remaining in traces in the product (“neutral wash”).
The aim of these washing steps is to obtain not only a pure product but also very little wastewater per metric ton of product, where the washed-out impurities are present in the wastewater in such a form that they can be disposed of inexpensively.
To minimize the amounts of water required for this wash, the wash can, for example, be carried out in countercurrent in such a way that the water used for the neutral wash is, after addition of bases, used in the alkali wash (cf., for example, A. B. Quakenbush et al., The Olin Dinitrotoluene (DNT) Process, Polyurethanes World Congress 1993, Publish.: Technomic Lancaster, pages 484 to 488) or that the acid wash is carried out using a minimal amount of water, so that a concentrated acid which can be recirculated either directly or after further concentration to the nitration is obtained.
Thus, EP 0 279 312 B1, EP 0 736 514 B1 and EP 1 780 195 B1 describe processes by means of which the mineral acids still suspended and dissolved in the nitroaromatics after the nitration, e.g. sulfuric acid, nitric acid and nitrogen dioxide, are washed out in a plurality of stages and selectively and are recirculated to the nitration, so that no wastewater is obtained from the acid wash and has to be disposed of.
However, processes in which, in order to minimize the amount of wastewater to be treated, no acid wash is carried out but instead only an alkaline wash and a neutral wash, as described, for example, in Kirk-Othmer, Encyclopedia of Chemical Technology, 4th ed., Vol. 17, pages 136 to 138, or in U.S. Pat. No. 4,091,042 A, have also become known.
Apart from minimizing the waste streams, a further aim is to minimize the technical outlay required for the wash (e.g. by the technology used for washing being specifically matched not only to the washing stage but also to the product to be washed).
As washing apparatuses, mixer-settler units (cf., for example, EP 1 593 654 A1) in which the mixing part is usually a stirred vessel (cf., for example, Ullmann's Encyclopedia of Industrial Chemistry, 5th ed., Vol. B 3, pages 6.19 to 6.21; M. Baerns et al., Technische Chemie, Verlag Wiley-VCH 2006, pages 352/352) are usually used in the individual washing stages for washing the nitroaromatics to be purified. Thus, the German patent DE 1 135 425 describes an arrangement of mixers and settlers which allows even nitroaromatics which are crystalline at room temperature, e.g. DNT, TNT or NCB, to be washed in liquid form at elevated temperatures with minimization of the outlay for heating. However, centrifugal pumps and static mixers have also been used as mixers (cf., for example, the documents U.S. Pat. No. 3,221,064 A or EP 1 816 117 B1).
However, the use of the mixer/settler technology (cf., for example, FIG. 1) is complicated and expensive. Due to the unavoidable carryover in the case of continuously operated stirred vessels as mixers, it is, especially in the removal of nitrophenols or nitrocresols when these are present in high concen-trations in the crude nitroaromatic, necessary to work in a number of stages and preferably in countercurrent in order to obtain the low content of impurities which is desired for the further processing of the nitroaromatic (e.g. a content of nitrophenols of less than 10 ppm, preferably from 2 to 3 ppm). A wash in multistage extraction columns is also technically complicated and expensive and not very effective. In addition, the generation of large exchange areas for a two-phase mixture in a short time for effective mass transfer followed by a rapid chemical reaction can be achieved neither in a stirred vessel nor in extraction columns.
J. M. Coulson, F. E. Warner, “A Problem in Chemical Engineering Design: The Manufacture of Mononitrotoluene”, a publication by “The Institution of Chemical Engineers”, 56, Victoria Street, London S.W.1, 1949, pages 25/26, describes a triple wash of the MNT using a washer of the Holley-Mott (mixer/settler) type, in which the acid wash and the alkaline wash is carried out in countercurrent in at least two stages in order to achieve sufficient removal of the acids and nitrocresols dissolved or suspended in the MNT.
In the Canadian patent CA 1 034 603, a four-stage acid wash in countercurrent is proposed in order to wash out the nitric acid and sulfuric acid dissolved and suspended in the crude DNT.
U.S. Pat. No. 4,091,042 A describes a four-stage wash using sodium carbonate in countercurrent for removing all acidic components from crude nitrobenzene, e.g. entrained sulfuric acid and the dinitrophenols and picric acid dissolved in the nitroaromatic down to 2000 ppm and obtain the desired purity.
EP 1 816 117 A1 describes a four-stage neutral wash in countercurrent using four stirred vessels and the associated separation apparatuses (known as “mixer/settler technology”) in order to reduce the still too high content of nitrophenols after the alkaline wash from about 50 ppm to a residual content of about 2 ppm. However, even when the stirred vessels are replaced by centrifugal pumps as mixing devices, three stages are still required to obtain a residual content of nitrophenols in the resulting nitrobenzene of 3 ppm.
U.S. Pat. No. 4,994,242 A discloses that static mixers are not suitable as mixing device in two-phase systems on the industrial scale alone to produce optimal dispersion of the two mutually immiscible phases in one another. Thus, EP 1 816 117 B1 describes the use of a static mixer for the alkaline wash; the nitrobenzene treated therewith still contains more than 50 ppm of nitrophenols which have to be brought down to about 2 ppm by means of a complicated multistage neutral wash.
As has been explained for an acid wash in EP 1 780 195 B1, the washing of nitroaromatics is a complex operation. Apart from generation of a sufficiently large exchange area between organic phase and washing phase (usually water) in order to achieve optimal transition of the impurity to be removed from the organic phase, the effectiveness of a washing stage depends on the partition equilibria of the impurity between organic phase and washing medium and also on whether the impurity extracted from the organic phase is stable as such in the washing medium or is withdrawn from the partition equilibrium by a subsequent reaction.
Thus, nitrogen dioxide reacts with water after transition from the organic phase into the aqueous phase so as to disproportionate into nitric acid and NO according to equation (1):3NO2(=3/2N2O4)+H2O→2HNO3+NO  (1)
Both the transition of the nitrogen dioxide from the organic phase, probably as dimer, and also the reaction of the nitrogen dioxide (as N2O4) with water are comparatively slow reactions compared to a neutralization, so that time is required for removal of the nitrogen dioxide from the organic phase by means of a wash with subsequent chemical reaction.
On the other hand, in the case of acids such as sulfuric acid, nitric acid or the weakly acidic nitrophenols, the dissociation of the acids into hydronium ions and the associated anions which occurs in the washing water (equation 2) or the neutralization which occurs in the presence of alkali (equation 3) is a very rapid process by means of which the washed-out impurities are withdrawn from the partition equilibrium between nitroaromatic and washing water and are then found in anionic form only in the washing water.H2SO4+H2O→H3O++HSO4−  (2)NO2Ar—OH+NaOH→NO2Ar—O−Na++H2O  (3)
As a result of this rapid neutralization of anion-forming materials in the alkaline washing medium, it is to be expected that the extraction of these materials from the organic phase is essentially mass-transfer-controlled and the wash follows essentially the same kinetic laws as a mononitration, e.g. the nitration of benzene to form nitrobenzene.
The processes and plants known from the prior art for purifying crude nitrated products often do not operate with high efficiency or else not in a satisfactory way. Hitherto, excessively complex process sequences or operations have been associated therewith, and the desired purities are often not achieved, at least not with a justifiable outlay.